It is known in the prior art to provide a pipette gun to replace the dangerous and often prohibited practice of mouth pipetting liquid samples. A typical pipette gun comprises a hand-held unit in communication with a laboratory pipette at one end and connected to either a remote or local air pressure source at the other end. A valve located within the pipette gun regulates the flow of air through the gun and to the pipette to control either the intake or expulsion of liquid through the pipette. The operator regulates air flow to the pipette by depressing either the positive pressure trigger or negative pressure trigger on the pipette gun. The magnitude of the pressure is predetermined and controlled by the valve located within the pipette gun housing.
Some pipette guns are provided with a universal nose piece attachment for cooperating and communicating with pipettes of various lengths and diameters. It is necessary for practical use of such pipette guns to provide variable flow rates to accommodate the different pipettes. For example, while a low flow rate is preferred for precise metering of liquid samples in small pipettes, a low flow rate is inefficient for larger pipettes. The range of flow rates of the pipette gun valve thereby effectively limits the size range of pipettes with which the pipette gun may be used practically.
The advantages of a pipette gun which can variably control the liquid flow rate through the pipette are recognized in the prior art. For example, it is known to variably control the air pressure at the pressure source by putting a speed control on the pressure pump. When an operator increases the motor speed, however, the pump diaphragm jerks irregularly and causes a temporary irregular flow rate through the pipette.
It is known and recognized in Kenney U.S. Pat. No. 3,963,061, to provide a pipette gun having an adjustable valve for continuously variably controlling the pressure applied to the pipette from a constant pressure source and, thus, continuously variably controlling the liquid flow rate through the pipette. The operator controls the liquid flow rate through the pipette by limiting the extent to which the trigger on the gun is depressed. In this manner the operator can rapidly fill or void a major portion of the pipette by depressing the trigger fully and then slowly meter the pipette by depressing the trigger slightly.
The type of pipette gun provided by Kenney improves on the prior art and is useful for many applications. The full range of liquid flow rates is achieved over the action path of the trigger. Due to the wide range of liquid flow rates and the limited action path of the trigger, however, slight deflections of the trigger in Kenney produce significant changes in the liquid flow rate. Limiting the range of flow rates for a particular trigger path of action makes the trigger less sensitive but limits the usefulness and efficiency of the pipette gun. It is therefore an object of the present invention to provide a pipette gun having a wide range of liquid flow rates which can be easily and precisely controlled by the pipette gun triggers.
Additionally the pipette gun in Kenney has limited use where extremely precise metering of small pipettes is required. The piston-like movement of the valve stem within its valve chamber causes small pressure changes in the pipette as the valve moves into and out of registry with pressure source. These slight pressure fluctuations adversely affect the liquid level in the pipette after releasing or initially depressing the trigger of the pipette gun. This problem is amplified when using small pipettes. It is therefore another object of the present invention to provide a valve whose constructions eliminates the "piston-effect" during operation.